Device for controlling temperature



Oct. 13, 1942. .1. MCAFEE DEVICE FOR CONTROLLING TEMPERATURE Filed Oct.24, 1940 www Patented Oct. 13, 1942 DEVICE FOR CONTROLLING TEMPERATUREJerry McAfee, Chicago, Ill., asslgnor to Universal Oil Products Company,Chicago, lll., a corporation of Delaware Application october zi, 1949,sensi No. 362,512

a claims. (ci. ca -zas) This invention relates to improvements in anapparatus for automatically regulating and controlling temperatures andmore specifically is directed to the regulation of temperature in areaction zone in response to changes in temperature within the zonewhich are transmitted as electrical impulses to a temperature measuringdevice operatively associated with the reaction zone.

In many catalytically promoted hydrocarbon conversion reactions, suchas, for example, catalytic cracking, dehydrogenation, polymerization andother reactions, heavy hydrocarbonaceous materials resulting from theconversion reaction are deposited in a mass of granular contact materialwhich catalyzes, directs or otherwise favorably influences the reaction.Thesevdeposits eventually impair the actvity of the contact material andits activity can only be maintained at an eiiicient level byperiodically burning the deleterious combustible deposits therefrom.This is ordinarily accomplished by passing a stream of hotoxygen-containing gases through the contact mass after the ow ofhydrocarbon reactants to be converted has been discontinued or thestream thereof diverted to another similar reaction zone wherein theconversion reaction is continued in the presence of another mass of thesame active contact material. 'I'he gaseous medium ordinarily employedfor reactivation comprises combustion gases or other relatively inertgas containing a regulated amount of air or oxygen. The reactivating gasstream is introduced into contact with the contaminated material to bereactivated at a suinciently high relatively inert gasesto serve as thereactivating gas mixture, thereby controlling or limiting thetemperature developed in the bed of catalyst or l contact materialundergoing reactivation.

trolling the introduction of air or oxygen and thus controlling thetemperature of the material undergoing reactivation which mayconveniently be employed in conjunction with conventional temperatureindicating or recording instruments now in common use. This involves theemployment of essential parts and operating functions of suchinstruments as motivating means for my control system.

'Another feature of the invention resides in the provision thereofwhereby the highest temperature prevailing at any of a plurality ofpoints in the mass of contact material undergoing reactivationdetermlnes {the amount of oxygen present in the reactlvating gas stream,any lower temperature prevailing at other points in the contact massbeing ineffective. This is particularly advantageous since in` mostreactivating operations of the nature above described a zone ofrelatively rapid combustion and resulting high temperature progressesthrough the mass or bed from a point adjacent the inlet of thereactivating gas stream to a point adjacent its outlet as thereactivating operation progresses. Thus the zone of highest temperaturechanges during the reactivating operation and a temperature ,measured ata single point in the bed or mass sensitive elements at spaced pointsthroughout the length o1' the bed of contact material, periodicallytransmitting the impulses generated at each of the thermocouples to atemperature indicating or recording instrument and translating theimpulse of greatest magnitude which occurs during each complete cycle oftemperature indications or recordings into a force which adjusts thequantity of air or oxygen admitted to the reactivating gas stream, I amable to deiinitely limit the maximum temperature prevailing ,at any ofsaid spaced points and thus prevent excessive heating of the contactmaterial in any portion of the bed.

Other objects and advantages of my invention will be apparent from theaccompanying drawing and the following detailed description thereof.

In the drawing, Figure 1 is a diagrammatic illustration of the apparatuscomprising my invention.

Figure 2 is a schematic View of the electrical circuits employed, thevarious parts of the circuit being shown'diagrammaticaliy.

There are many temperature recording instruments on the market which areemployed to give a visual indication c-r written recordation oftemperature conditions under predetermined circumstances. Most of theseinstruments employ the same general principle of operation. For example,one well known instrument, the "Tagliabue Celectray, has a block whichmoves in translatory motion in response to the temperature of o-ne ormore thermocouples with which the instrument is connected. This blockcarries a printing head which records, on a cooperatively moving webofpaper, the temperature of a predetermined thermocouple. In this case,the essential movement of the instrument which would be employed in myinvention would be the movement of the block in response to variationsin temperature.

When, as is usually the case in the use of such instruments, thetemperatures of a plurality of thermocouples are to be individuallymeasured'and recorded, an automatic selector switch is provided whichselectively closes the circuit of a predetermined thermocouple with theinstrument in timed relation with the movement of the Web of paper uponwhich the respective thermocouple temperatures are recorded. When aninstrument is connected with a predetermined number of thermocouples,the operation of once recording the `temperature of each thermocouple isknown as a cycle, or recording cycle."` This cyclic movement is also oneof the essential movements of the conventional temperature recordinginstrument which I employ in my invention, as Will be hereinafter morefully described.

Referring in detail to the drawing, the reactor tube or chamberindicated by the reference numeral I contains a bed of contact materialor catalyst, not illustrated, of the type which requires periodicreactivation in the manner above described. During pro-cessing Aof thehydrocarbons or other materials to be converted they are admitted to thereaction zone at a suitable temperature through pipe 4, valve 5 andinlet pipe 2. The reaction or conversion of the reactants occurs duringtheir passage through the reaction zone in contact with the bed ofcatalyst cr contact material disposed therein and the resulting fluidconversion products are discharged from the reaction zone through outletpipe 3 and the communicating pipe 6 containing valve l.

During the period of operation in which the catalyst or contact mass isemployed for promoting, directing or otherwise favorably affectingconversion of the reactants admitted through pipe 4, deleterious heavyconversion products l formed incidental to the desired reaction aredeposited on the contact material and eventually retard its activity tosuch an extent that they must be removed therefrom if the catalyst orcontact material is to be eiciently further employed. When thisstage inthe operation is reached, valve 5 in pipe Il is closed, the stream ofreactants to be converted preferably being diverted to another similarreaction zone containing fresh or freshly reactivated contact materialandwherein the conversion reaction is continued. Preferably, after valve5 is closed, valve 'l in pipe 6 remains open for a suicient period oftime to permit the removal of fluid reactants and conversion productsfrom the reaction zone, such as, for example, by purging the reactorwith inert gases or the like. The hot oxygencontaining reactivating gasstream is then admitted to the reactor, as will be presently described,to burn the deleterious deposits from.

the contact mass and the resulting spent or partially spent reactivatinggases and combustion products are discharged from the reactor throughoutlet pipe 3 and through the communicating pipe I0 containing valve II,the latter having been closed during the previous processing phase ofthe operating cycle.

The reactivating gas stream employed in this particular instancecomprises substantially oxygen-free ue gases or the like, admitted topipe I3 through pipe I4, and regulated amounts of air admitted t0 pipeI3 through line I5 and the valves I5 and I6. Pipe I3 communicates with aheating coil I2 in heating zone I2 and the mixture of combustion gasesand air supplied to the heating coil through pipe I3 are therein broughtto a temperature suitable for initiating combustion of the deleteriousconversion products deposited in the contact mass upon contact of thereactivating gas stream therewith. The A hot reactivating gas stream isdirected from coil I2 through pipe 8, valve 9 and inlet pipe 2 to thereaction zone during reactivation of the contact material. Valve 9, likevalve II, is closed during the processing phase of the operating cyclein reactor I. The heating zone I2 may comprise a furnace setting, heatvexchanger or other conventional heating means capable of increasing thetemperature of the reactivating gases to the desired value and ispreferably operated by any suitable form of conventional controlequipment, not illustrated, to maintain a substantially constanttemperature in the reactivating gas stream entering the reactor.

l Valve I5 in pipe I5 may be a hand-operated valve or a valve controlledby suitable time cycle equipment or the like of conventional form, notillustrated, and is normally fully open While reactivation is takingplace in reactor I and fully closed during the' processing phase of theoperating cycle in reactor I. Its principal function is to permitpurging of the reactor of combustible fluid reactants and conversionproducts following the processing phase of the operating cycle andsimilar purging of the reactor of oxygen-containing gases following thereactivating phase of the operating cycle, purging being accomplished byleaving valve I5 closed during the purging period and supplyingsubstantially oxygen-free combustion gases or other relatively inert gasto the reactor through pipe Id, heating coil I2', pipe S, valve 9 andinlet ypipe 2.

Valve I@ in pipe I5 is a motor operated valve which, while valve I5 inthis line is open, functions to regulate the quantity of air or oxygenadmitted to pipe I3 and therein commingled with the relatively inertcomponents of the reactivating gas stream. The opening through valve l5is regulated through the control mechanism provided by the invention inresponse and in inverse relation to the maximum temperature Iprevailingin the bed of contact material undergoing reactivation at any one of aplurality of spaced points therein. Its operation will be hereinafterdescribed in greatest detail in conjunction with the description of theentire control circuit and mechanism.

Since the zone of highest temperature in the bed of contact materialundergoing reactivation will vary as reactivation progresses, aplurality of thermocouples designated by the reference numerals I'I, I8,I9, 20, 2I and 22 are positioned at spaced points within the bed ofcatalyst or contact material throughout the length oi' reactor I. Thewires leading from the thermocouple junctions pass, in the particularcase here illustrated, through the ends o! inlet and outlet pipes 2 and3 which latter are sealed to prevent the escape of iiuids therethrough'from the reaction zone. These lead wires or electrical connections aredesignated by the reference numeral 23 and are connected by well knownmeans, not illustrated, to a suitable conventional form of tem-Iperature measuring or recording instrument oi the type hereinbeforementioned, such as, for example, the Tagliabue Celectray, wherein theimpulses irom the thermocouples are translated into temperatureindications or recordings.

Referring now to the control mechanism which operates valve I6, thereference numerals 24 indicate spaced frame members which may comprise aportion of the recording instrument employed or which may comprise theframe oi' a separate instrument adapted to contain the essential workingelements of my apparatus. A rod 25 is ixedly secured at its oppositeends to the opposite frame members 24, said rod serving as a guide for ablock 26, which may comprise the recording head of the high temperaturerecording meter employed. As has been hereinbeiore described, most ofsaid recording meters are constructed along the same principle, the head26 being moved in response to diterences in temperature or thethermocouples I1 to 22 inclusive. For purposes of illustration the head28, during a recording cycle, will move to the right, as viewed in Fig.1, when the temperature of the selected thermocouple is higher than thetemperature of the last thermocouple measured, and will move to the leftwhen the temperature of a thermocouple being measured is less than thetemperature of the thermocouple previously measured.

Heretofore, utilizing a conventional recording instrument, the movementsof the head 26 were noted, or the chart which was imprinted by the head26 was employed to indicate to the operator whether it was necessary toincrease or decrease the temperature oi' the catalyst within the chamberI. To increase or decreasesuch temperature a manually operated valve inpipe I was manipulated in order to increase or decrease the air supplyto the gas heater I2. My invention, hereinafter described, provides anautomatic means for properly controlling the temperature within thecatalyst chamber l Without manual observation or operation.

A bar 21 is positioned between the spaced frame members 24 andappropriately mounted at its opposite ends to said frame members. Thelower surface of the bar is provided with ratchet teeth 28. A travelinghead 29 is slidably mounted upon bar 21, said head having an extension30 which is slidably positioned upon the rod 25. A coil spring 3! isanchored, as at 32, to one of the frame members .24, the opposite endthereof being anchored, as at 33, to the block 29, the function of thecoil spring being to normally urge block 29 to the left, as viewed inFig. l.v An adjustable stop 34 is mounted upon bar 21 and functions asan abutment for the block 29, limiting its travel in response to thespring 3|.

The arrangement is such that when the head 26 of the recordinginstrument moves to the right, it contacts extension 30 ofthe -block 29and thus moves said block against the tension of spring 3| to the right.A lever 36 is pivotally mounted upon the block 29, as at 31, said leverhaving a pawl 38 at one end which is adapted to engage with the ratchetteeth 23. The opposite end of the lever 36 extends downwardly, asindicated at 39 in Fig. -1.

A blade spring (not shown) normally tends to rock lever 36 in aclockwise direction. Hence, when the head 26 moves to the right inresponse to the measurement of a thermocouple within the chamber I whichis higher than the thermocouple previously measured, said head moves theblock 29 to the right. However, when the head 26 moves to the left, thatis, when the temperature of a thermocouple is measured which is lowerthan the temperature previously recorded. the block 29 is prevented fromfollowing the head 26 under the iniluence of spring 3| by the engagementof the pawl 38 with the ratchet teeth 23.'

It is clear, therefore, that the position of the block 29 is dependentupon the highest temperature of a predetermined thermocouple-which ismeasured during a recording cycle. This movement is employed as theprimary actuating force which sets into movement other agencies forcontrolling the valve I6.

A disc 40 is mounted upon a xed hub 4I adjacent the bar 21. The disc 40is adapted to be rotated in timed relationship with the mechanism of theconventional recording instrument which moves the paper web or chartpast the printing head 26. 'I'he arrangement is such that the disc 40makes onecomplete revolution for a recording cycle, that is. acyclewherein the temperature of the thermocouples I1 to 22 are eachseparately measured once. A pin 42 is carried upon the disc 40 adjacentits periphery and extends at right angles tothe surface of the disc. Abar 43 having offset ends 44 is carried at its opposite ends by thespaced frame members 24. 'I'he bar 43 is rockable about pivots 45 whichserve as journals for said bar.

The operation is such that when the disc 40 has rotated a predetermineddegree, the pin 42 contacts the lower surface of the bar 43. As therotation of the disc 40 in a clockwise direction continues, the pin 42acts to rock bar 43 about the journals 45. In doing so, bar 43 contactsthe lower end of the depending member 39 of the lever 36, therebyrockirm said lever in counter-clockwise direction and disengaging thepawl 38 from the ratchet teeth 28. Disengagement of the pawl leaves theblock 29 free to move under the iniluence of the spring 3l and saidspring causes the block to move to the left until it abuts against thestop 34 or the extension abuts against the head 26. The apparatus isthen in condition to commence another cycle of recordations.

A resistance wire 46 is strung between the opposite frame members 24 andis insulated from said frame members by means of insulators 41. A lug 48extends downwardly from block 29 and a trolley wheel 49 is journalled inthe lug 48, saidI trolley wheel running along and making electricalcontact with the wire 46. A wire 56 is grounded upon the block 29 andmakes electrical contact with the trolley wheel 49. The opposite end ofsaid wire is connected to an instrument 5l of the galvanometer type.Although a simple galvanometer movement would function, it is to beunderstood that the instrument 5I may comprise a polarized relay or anelectronic tube system which is responsive to currents passed inopposite directions.

The opposite end of the instrument 5I is connected to wire 52 which, inturn, is connected to an arm 53 having a contact point 54. Arm 53 ispivotally mounted at 55 and the arrangement isvsuch that the contactpoint 54 may be swung to make contact with a variable resistance 56. Uneend of the resistance 56 is connected by means of wire 51 to a pointadjacent the end of the wire 46. The opposite end of the resistance 56is connected -by means of wire 58 to a battery 59 or other source of E.M. F. A wire 60 also connects with wire 58, the opposite .end of thewire 60 being connected to the opposite end of the resistance wire 46.

A segmental electric contact member 6I is mounted upon the face of disc40. An arm 62 extends radially from a ring 63 whichcomprises the outerportion of hub 4I. The outer end of said arm is positioned immediatelyover the path of travel of the contact member 6I and during a portion oftherevolution of the disc 40, the arm 62 makes electrical contact withthe member 6I. A wire 64 connects wire 51 with the ring 63 and a wire 65is connected at one end to the battery 59 and at the opposite end to acontact point 66 which is positioned adjacent the path of travel of thecontact element 6I, whereby electrical contact is made between the point66 and the member 6I during a portion of travel of` the disc 46, thearrangement being such that connection is made by the member 6I betweenthe arm 62 and contact point 66.

Referring particularly to Fig. 2, the variable resistance 46 correspondsto the wire 46; the movable contact point'49 corresponds to the Ytrolleywheel 49; wire 56 corresponds to wire 56 and connects with instrument5I' which corresponds to the instrument 5I. The resistance 56'corresponds to resistance 56, and contact point 54' corresponds to thecontact point 54, the contact point 54' being connected to theinstrument 5I by means of wire 52'. Battery 59 corresponds to the source-of electromotive force 59, and switch 62' corresponds to thearrangement comprising arm 62, contact member 6I and contact point 66.

It can readily be seen from the schematic diagram that a bridgearrangement vis provided. In this arrangement when switch 62' is closed,that is, during that interval when the member 6I bridges arm 62 andcontact point 66, a current will be passed through the circuitcomprising wire 50', instrument 5I' and wire 52' if the positions of thecontact members 49' and 54 are such that a balance of electromotiveforce is not created. When the contact points 54' and 49 are so adjustedalong the variable resistances 46 and 56' as to balance theelectromotive forces, no current will ow through the instrument 5I Itcan readily be seen that' three vconditions may exist in the instrument5V; (1), the current may pass through said instrument in one direction;(2), the current may pass through the instrument in an oppositedirection, or (3), no`current may pass through the instrument.

Another segmental contacting element 61 is positioned upon the disc 40in circumferential spaced relationship with respect to the member 6i.The reference numeral 68 indicates a source of electric current, a wire69 connects the center 10 of the hub 4I to one side of the source ofelectric current. An arm 1I is mounted upon the central member 10 andextends radially from said central member, said arm being angularlyspaced with respect to arm 62. The end of arm 1I is positioned over thepath of travel of the segmental member 61 and during that period ofrotation when the segmental member 61 is beneath arm 1I, an electricalconnection is established between the wire 69 and the segmental member61. The galvanometer or other current indicating instrument 5I isprovided with an arm 12, said arm being swingable about the center 13.When current passes through the instrument 5I in one direction, the armwill swing in a predetermined direction about the center 13. Whencurrent is sent through the instrument 5I in the opposite direction thearm 12 will swing in the opposite direction. When no current passesthrough the instrument 5I, the arm 12 is disposed in normal orintermediate position. A wire 14 connected at one end to the source ofelectric power is connected at its opposite end to arm 12. The arm 12,when in normal or intermediate position, is disposed midway betweencontact points 15 and 16. When a current is passing in one directionthrough the instrument 5I the arm 12 makes contact with the point 15 andwhen passing in the opposite direction the arm 12 makes contact with thepoint 16.

The contact points 15 and 16 connect respectively by means of wires 11and 18 to opposite ends of a `fleld winding 19 of an electric motor 80.A wire 8l Vconnects the central. portion of the field Winding 19. to acontact point 82 which is disposed in the path of travel of thesegmental contact member 61. A pinion 83 is mounted upon the shaft oimotor 80, said pinion meshing with a spur gear 84. A'lever arm 85 isrigidly mounted upon the gear 84 and is pivotally connected as at 86 tolink 81. Link 81 intermediate its length is pivotally connected to arm53 as at 88, the opposite end of said link being connected to valve I6.

The arrangement is such that during the period of rotation of the disc48 whenA arm 1I is in contact with the segmental element 61 and thecontact point 82 also makes electrical connection with the segmentalelement, the wire 69 from the source of electrical power 68 is connectedwith wire 8l which, in turn, is connected to the center of the fieldwinding 19. If current passes through the instrument 5I the arm 12 makescontact with either point 15 or point 16 thereby passing current in oneor the other direction ,through the field winding 19. In this manner themotor may be driven in a clockwise or counter-clockwise directionwhereby link 81 is moved to simultaneously open or close valve I6, orincrease or decrease the communicating opening in said valve and movethe arm 53 to change the value of the resistance on each side of thecontact Point 54.

In employing my invention the recording cycle will start when the pin42is immediately beneath the bar 43 and is moving downwardly. In thisposition of the disc 40 the segmental elements 6I and 61 are removedfrom the contact points 66 and 82, and the arms 62 and 1I shall remainstationary. In accordance with the operation of the conventionalrecording instrument the temperatures of the individual thermocouplesI1, I8, I9, 20,'2I and 22 are separately measured in sequence. In viewof the fact that the bar 43 is hanging in its lowermost position, thelever 36 is free to rotate and permit the pawl 38 to engage the ratchetteeth 28. Consequently, as the different thermocouples are brought intocircuit, the head 26 will move along the rod 25. In moving to the right,as viewed in Fig. 1, the head will move the block *29, but movement tothe left will not cause a corresponding movement of the block 29. Hence,said block occupies the position proportional to the highest temperatureindicative of the highest temperature thermocouple. After the recordingcycle has been completed, the disc 40, moving in timed relationship withthe conventional recording instrument mechanism will have moved to sucha position that the segmental contact elements 6i and El will passbeneath the arms 62 and li? respectively. At this period of operationthe bridge circuit will be completed and an unbalanced condition exists,as determined by the position of ther trolley wheel 4d upon the wire 4t,the arm 'l2 will move into contact with either the point 'Iii or 1%.Simultaneously, with completing the bridge circuit, the motor circuitwill also be completed by the positional relationship of the arm l,segmental contact El and contact point 82. Hence, current will be passedthrough the field winding '9 inpone or the other direction therebycausing motor 80 to rotate clockwise or counterclockwise as the casemaybe. Rotation of the motor in one direction or the other causes anincrease or decrease of flow of air to the gas heater l2.Simultaneously, arm 53 will be moved along the variable resistance 56 tocut in or cut out sufficient resistance to balance the bridge. When thebridge is balanced, that is, when no current flows through theinstrument 5i, the arm 12 seeks its normal position and, hence, themotor circuit is broken.

During the period when the motor B operation and the valve I6 is beingmanipulated and l arm 53 is being moved, the disc 40 will have rotatedto such a position that the pin 42 is immediately beneath the bar 43 andis rising, that is, the position shown in full lines in the drawing.Further rotation of the disc 40 breaks both of the electrical circuitsand simultaneously the pin 42' functions to raise bar 43 into contactwith the lower depending end of lever 36, thereby releasing the pawl andpermitting the block 29 to return either to the stop 34 or to theposition of the head 26. The entire apparatus is then in position torepeat the operation above described.

Of course, it is to be understood that I do not wish to be limited tothe specific apparatus herein described. In the rstinstance, theapparatus is shown highly diagrammatically and in no sense is'itintended to be contemplated as being proportional. Many ot theinstruments shown such as the galvanometer 5l may be replaced by similardevices such as a polarized relay or the like. The reversing motor isshown as one wherein the eld of the motor is center tapped. Of course,any suitable reversing motor may be employed. The number ofthermocouples employed will depend upon the type of conventionalrecording meter with which my apparatus is used. Broadly, my inventioncontemplates employing and taking advantage of the fundamental movementsof a conventional temperature recording instrument and, hence, I do notwish to be limited to the use of my invention with any specifictemperature recording instrument.

I claim as my invention:

1. A device for controlling the temperature of a catalyst bed during theperiod of reactivating the catalyst by controlled oxidation whichcomprises, means movable in response to variations in temperature ofspaced points of a catalyst bed, a Ioliower moved progressively by saidmeans as the temperature increases, means for holding said follower in aposition corresponding to the maxi mum progressive moment of saidmovable means, reversible electrical means for actuating a valve forcontrolling the admission of an oxygen-containing gas to the catalystbed, a resistance bridge, means carried by said follower in contactwith, and forming the juncture oi two arms of said bridge, wherebymovement of said follower changes the resistance ratio of said two armsto unbalance said bridge, means actuated by movement of said electricalreversing means for varying the resistance ratio of the other two armsof the' bridge to rebalance the bridge, and means connected between saidlast mentioned means and said follower contact means for controlling thedirection of current through said reversible electrical means.

2. In combination, with a device for indicating by the movement or ameasuring head, the temperature of a plurality of spaced points in acatalyst becl, measured sequentially in cyclic manner, of a followermoved forwardly by said measuring head as the temperature of said pointsincrease,

.means for holding said follower in a positioncorresponding to themaximum forward movement of said measuring head in a predeterminedmeasuring cycle, reversible electrical means for actuating a valve tocontrol the admission of an oxygen-containing gas to the catalyst bed, aresistance bridge, means carried by said follower in contact with, andforming the junction of, two arms of said bridge, whereby movement 'ofsaid follower changes the resistance ratio of said two arms to,unbalance said bridge, means actuated by movement of said electricalreversing means for varying the resistance ratio of the other two armsof the bridge to re-balance the bridge, and means connected by the saidlast mentioned means and. said follower contact means for controllingthe direction of current through said reversible electrical means.

3. In combination, with a device for indicating by the movement of ameasuring head, the temperature of a plurality of spaced points in acatalyst bed, measured sequentially in cyclic manner, of a followermoved forwardly by said measuring head as the temperature of said pointsincrease, means for holding said follower in a position corresponding tothe maximum forward movement of said measuring head in a predeterminedmeasuring cycle, reversible electrical means for actuating a valve tocontrol the admission of an oxygencontaining gas to the catalyst bed, aresistance bridge normally inoperative during said measuring cycle,means carried by said followe'r in con tact with, and forming thejunction of, two arms of said bridge, whereby movement of said followerchanges the resistance ratio of said two arms to unbalance said bridge,means actuated by movement of said electrical reversing means forvarying the resistance ratio of the other two arms of the bridge tore-balance the bridge, means connected by the said last mentioned meansand said follower contact mea-ns for controlling the direction ofcurrent through said reversible electrical means, and means forrendering said bridge operative at the end of each cycle after thetemperatures of all of said points have been measured.

JERRY MCAFEE.

